1. Field of the Invention
The present invention relates to aluminum titanate having high strength which is stable at high temperature especially even at high temperature in a reducing atmosphere.
2. Description of the Prior Arts
The ceramic compositions have various characteristics but have disadvantages of relatively low thermal shock resistance, whereby the usages are limited.
It has been well known that the most effective method for improving the disadvantage of low thermal shock resistance is to impart low thermal expansion for the ceramics. Various proposals have been made. The ceramics having low thermal expansion such as lithium compounds e.g. .beta.-spodumene (Li.sub.2 O.Al.sub.2 O.sub.3.4SiO.sub.2) and aluminum-magnesium silicate (2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2) and aluminum titanate (Al.sub.2 O.sub.3.TiO.sub.2) are used in suitable shape. The .beta.-spodumene and aluminum-magnesium silicate have been practically used as ceramics for tablewares and gas burner devices.
However, .beta.-spodumene (melting point of 1430.degree. C.) and aluminum-magnesium silicate (melting point of 1470.degree. C.) have low melting point whereby they can be used for only limited special usages even though they are excellent materials. Accordingly aluminum titanate (melting point of 1860.degree. C.) has been studied as ceramics having low thermal expansion which can be used in the applications for high thermal shock resistance such as the iron steel manufacture.
The aluminum titanate has anisotropic characteristic in crystallography and has different coefficients of thermal expansion in crystallographic axes such as -26 .times. 10.sup.-7 .degree. C.sup.-1 in the a axis direction; 118 .times. 10.sup.-7 .degree. C..sup.-1 in the b axis direction; and 194 .times. 10.sup.-7 .degree. C.sup.-1 in the c axis direction.
The average coefficient of thermal expansion is not so small. However, in the sintered product obtained by bonding aluminum titanate particles having anisotropic characteristic, a large thermal stress (tensile stress) is internally formed in the directions of the b axis and the c axis because of the differences of coefficients of thermal expansion in the directions of crystallographic axes whereby many fine cracks are caused in the direction perpendicular to the b axis and the c axis so as to release stress. As the result, the coefficient of thermal expansion of the sintered product is mainly dependent upon the coefficient of thermal expansion in the a axis direction whereby aluminum titanate imparts remarkably low coefficient of thermal expansion. On the other hand, the strength of aluminum titanate is low because of the internal fine cracks.
The relation of the low thermal expansion and low strength of the aluminum titanate sintered product has considered as mentioned above.
There is the other problem of decomposition of aluminum titanate at high temperature which prevent practical applications of aluminum titanate sintered products.
The decomposition has been considered that Al.sup.+3 sites in the aluminum titanate crystals are remarkably larger than the ionic radius of Al.sup.+3 whereby Al.sup.+3 are taken out from the sites at high temperature. As the result, the amount of Al.sub.2 O.sub.3 gradually increases and the coefficient of thermal expansion gradually increases. The Ti.sup.+3 formed by reducing Ti.sup.+4 are entered into the vacancies formed by taking out Al.sup.+3. Accordingly, when aluminum titanate is used at high temperature in the reducing atmosphere, the change of the lattices may be easily caused.
It has been studied to overcome the disadvantage of the decomposition at high temperature which is fatal defect among the above-mentioned two disadvantages of the lowering of strength and the decomposition at high temperature.
It has been proposed to inhibit the decomposition of aluminum titanate at high temperature by substituting a part of Al.sup.+3 sites with Mg.sup.+2, Fe.sup.+3 or Cr.sup.+3 by solid-solubilizing MgO, Fe.sub.2 O.sub.3 or Cr.sub.2 O.sub.3 into aluminum titanate (U.S. Pat. No. 2,776,896; Japanese Patent Publication No. 26688/1967 and Japanese Unexamined Patent Publication No. 23113/1977). The ionic radius of the ions has only slight larger than the ionic radius of Al.sup.+3 whereby the effect for inhibiting the decomposition at high temperature is not enough high.